The aim of this project is to study ultrastructural features at the supramolecular scale in glial cells in the zones where mitochondria and endoplasmic reticulum adjoin. Interactions between these two types of organelle are associated with calcium signaling, through which glial cells communicate with each other during central nervous system development and synaptic activity. To preserve glial networks in their physiological state, high-pressure freezing has been applied. Cultured cells are first grown on specially coated gold specimen carriers, which are then frozen at a pressure of >2000 atmospheres at liquid nitrogen temperature to avoid ice crystal damage. Specimens are then freeze-substituted with osmium fixative before plastic embedding, sectioning and staining. The zones of interest in the glial cells are being imaged in three dimensions using electron tomography in a digital energy-filtering transmission electron microscope.